MEMS (Micro-electro-mechanical systems) accelerometer has been widely used in the automobile application, such as electronic stability control (ESC), adaptive cruise control (ACC), airbag systems and collision detection. The MEMS accelerometer even finds more applications in consumer electronics, for instance, the cell phone, PDA, digital still camera (DSC), and video games. Thus, the requirement of multi-axes accelerometer is rapidly increasing.
Presently, a capacitive-based type sensing approach has been widely applied as an inertial sensor. There are several advantages for the capacitive-based type sensing approach. Capacitive-based techniques are inherently less noise than piezoresistance techniques because of the lack of thermal noises. In the differential capacitive sensing approaches, because the output signal is a function of the capacitance difference existing between stationary electrode and movable electrode, any temperature effect acts the same to both capacitors and is therefore cancelled out therebetween, so that the signal stability is further improved. In the past, there are various in-plane accelerometers utilizing surface micromachining, bulk micromachining and CMOS-MEMS technologies. The out-of-plane CMOS-MEMS accelerometers use electric routing technique as parallel vertical combs for capacitive sensing. However, the sensitivity is quite restrictive because of the area variation sensing scheme.
Recently, CMOS-MEMS out-of-plane accelerometer with fully differential gap-closing capacitance sensing electrodes is presented. A post-CMOS wet etching process is established to realize the accelerometer with sensing electrodes of the sub-micron gap in the out-of-plane direction. However, one of the main challenges for multi-axes accelerometer is how to detect the acceleration in the Z-axis (out-of-plane axis).
In early periods, the capacitive sensing is employed using overlap area variation between movable and stationary vertical comb electrodes, and Z-axis capacitive sensing with a torsional suspension has been demonstrated using the technologies of DWP (Dissolved Wafer Processing). Also a monolithic three-axis micro-G resolution silicon capacitive accelerometer system implemented by using the combination of surface and bulk fabrication processes is demonstrated. However, the fabrication process is complicated with high cost.
A three-axis capacitive accelerometer has been developed using SOI (silicon on insulator) wafer. However, the electrode design in Z-axis is not the differential sensing architecture. The above-mentioned researches suffer from either non-differential sensing or complicated fabrication processes in the Z-axis sensing. Recently, the Z-axis differential SOI accelerometer is developed in different type of Z-axis novel vertical comb electrodes. The novel vertical electrodes are fabricated using two masks and a time-controlled RIE (Reactive Ion Etch) process, and then provide electrodes in different movable and stationary heights. According to this design, Z-axis acceleration is differentially detected easily using a set of the novel vertical electrodes. Furthermore, differential capacitive three-axis SOI accelerometer has been demonstrated using the novel vertical combs. However, Z-axis accelerometer with the gap closing differential electrodes using SOI wafer has still not yet been reported.
Accordingly, in order to fill the gap of the above-mentioned deficiencies in the state of the art, the applicant provides a gap-closing differential capacitive sensing three-axis accelerometer on SOI wafer to solve the above deficiencies in the prior art and reduce the cost thereof.
In order to eliminate the drawbacks of the conventional techniques, the new concepts and the solutions are proposed in the present invention so as to solve the above-mentioned problems. The present invention is described below.